In recent years, to solve signal delay, faster transmission rates, and other problems due to higher packing density and performance of electronic systems, optical wiring that excels in packing density and parallelism has received attention.
Since an optical circuit essential to optical wiring has been expensive, an optical waveguide made of a resin material (such as a plastic material), which can be manufactured at low cost by using a simple method, is considered as a promising technology.
An optical waveguide is primarily formed of a core and a clad and typically has a configuration in which the clad surrounds the core. Reflection and refraction, according to which light propagates, are repeated in accordance with the difference in refractive index between the core and the clad as well as the angle of incidence of the light that impinges on the interface between the core and the clad.
Optical waveguides and their cores are made on a planar substrate because of ease of fabrication and cost. Optical wiring using an optical waveguide having such a planar structure, however, sometimes requires intersections of a plurality of cores. The optical loss at the intersections has to be minimized although optical loss at the intersections is inevitable.
In an optical waveguide having intersections, to reduce the amount of light leakage at the intersections, Patent Literature 1 describes a method for designing a two-dimensional planar structure in which the difference among three refractive indices n satisfies n1 (intersection of cores)>n2 (core)>n3 (clad) and a method for manufacturing an optical waveguide having the two-dimensional planar structure.
Patent Literature 1, however, does not describe a design in which each of the intersections of cores is taken as a three-dimensional structure and the characteristics of the interfaces present above and below the intersection of cores are also taken into consideration.
Patent Literature 2 describes an X-shaped intersection optical waveguide having an intersection of cores intersecting each other. In the optical waveguide, an effective refractive index discontinuity band (impurity region or groove) is formed at each portion where a core to be connected to the intersection is connected to the intersection.
The X-shaped intersection optical waveguide, however, does not have a configuration in which the discontinuity band divides the entire core but has a configuration in which only the thickness of an upper clad layer provided above the core is changed. Further, the purpose of the X-shaped intersection optical waveguide is only to allow the effective refractive index discontinuity band to function as a light blocking region to reduce the amount of light leakage at the intersection when light propagates therethrough.
Patent Literature 3 describes an optical waveguide having a lattice-shaped core layer formed therein and also having slits that extend parallel to the direction in which light travels and isolates an intersection in the core layer and a method for manufacturing the optical waveguide. In Patent Literature 3, the slits are provided to prevent optical loss.
Patent Literature 3, however, does not describe filling the divided portions with a clad material or does not give consideration of using the same material not only to fill the three-dimensionally divided portions but also to form the portions above and below the divided portions.